Optical sensor element

ABSTRACT

The invention relates to an optical sensor element, comprising indicators ( 2 ), selected from luminescence-active means that are of the same type or different, and indicator protectors ( 1 ), and to a sensor, comprising at least one such sensor element, an energy source that excites the luminescence emission of the indicators, and a detector unit, wherein the sensor element or sensor is suitable for detecting molecular oxygen in a gaseous or liquid medium and/or for determining the molecular oxygen content of a gaseous or liquid medium and at least one layer of the sensor element bearing the indicator protectors is designed in such a way that the diffusion rate of the molecular oxygen formed on the indicator protectors by means of the reduction of strong oxidants back into the medium is greater than the diffusion rate of molecular oxygen from the medium in the direction of the at-least-one layer bearing the indicator molecules.

BACKGROUND

1. Technical Field

This invention relates to a sensor element and a sensor comprising thistype of sensor element, where the sensor compounds (indicators)contained in the sensor element are protected against damaging orinactivating influences, such as highly reactive compounds. Theinvention further relates to the use of the sensor element and thesensor for determining an analyte in an environment which is aggressivefor the indicator.

2. Discussion of Related Art

Sensor structures containing excitable sensor compounds (indicators) aregenerally known. The sensor principle is based on the fact that theindicators are first converted to an excited energy state by the supplyof excitation energy. As the energy is released, e.g. in the form oflight of a certain wavelength, the indicators switch to a lower energylevel. The determination of an analyte in a sample is usually carriedout by measuring the energy emitted by the indicators, which issufficiently changed upon contact with an analyte to permit detection.

Different sensor types can be distinguished based on the form of theirexcitation and emission energy. The indicators of optical sensors can beexcited, for example, by a supply of light, or chemical or electricalenergy, although the emission always takes place in the form of light ofa definite wavelength. In the case of purely optical sensors, theexcitation and emission of the indicators takes place in the form oflight of a specific excitation wavelength (v₁₎ and emission (v₂₎wavelength. Optical sensors are used for example for the determinationof oxygen, halide, and heavy metal ions, carbon dioxide (CO₂₎ and of thepH value. The sensor principle may in this case be based on themeasurement of luminescence quenching, the change in luminescence decaytime, and/or the absorption of light waves.

At present, for the optical measurement of dissolved oxygen, sensors areused which are based on the property of certain luminescence indicators,whose luminescence of wavelength v₂ is excited by light irradiation of acertain wavelength v₁, and is dynamically quenched in the presence ofoxygen, such that oxygen causes the radiationless deactivation of theexcited state of the luminescence indicator.

Optical sensors are widely used for the reliable determination of ananalyte in complex media, because the measurement methods used here arecomparatively simple and require little equipment expense. However,conventional optical sensors have the disadvantage that the sensorelements do not provide adequate protection of the indicators theycontain from destructive influences, and in particular from reactivecompounds in the environment being analyzed. As a result, the servicelife of established sensor elements is especially limited when theindicators are exposed to conditions under which they are permanently orirreversibly inactivated.

In various applications, the medium to be analyzed contains, forexample, compounds which destroy the indicators due to a chemicalreaction. Thus, the problem with the current state of the technology isthat the optical sensor elements currently available for determiningmolecular oxygen either cannot be used or only with a very limitedservice life if the medium to be analyzed, such as waste water or waterfrom swimming pools, contains strong oxidants such as ozone, superoxideor hydroxyl radicals or chlorine or peroxide compounds used fordisinfection, the diffusion of which to the indicators in the sensorelement cannot be prevented, and contact leads to the oxidativeinactivation of the indicators. Due to the lack of protection for theindicators, conventional sensor elements are either not suitable or onlyto a very limited degree under these conditions.

There is therefore a need for optical sensors for the luminescencedetection of analytes in complex media, in which the luminescenceindicators of the sensor elements are effectively protected againstdestructive or inactivating influences from the environment beinganalyzed.

The sensors referred to in CN 102109488 A are only multilayeredelectrochemical oxygen sensors, the indicators of which are protected bya noble metal-doped layer with a catalytic function, which is arrangedbetween the medium and the indicator-bearing layer.

Optical sensors, the sensor elements of which are effectively protectedby the indicator-bearing layer or layers against attack by reactivecompounds, thus enabling the long-lasting determination of an analyte bymeans of luminescence measurement in a (chemical) environment that isaggressive for the indicators, are not known from the prior art, nor canthey be derived from it.

SUMMARY OF THE INVENTION

The present invention provides an optical sensor element, comprising

-   a) indicators that are selected from luminescence-active agents    which are of the same type or different, depending on the intended    use, and-   b) indicator protectors.

The indicator protectors of the sensor element according to theinvention are selected such that they protect the indicators fromdestructive or inactivating influences and particularly thosedestructive or inactivating influences to which the sensor element isexposed during its intended use.

The term “optical sensing element” in the context of the presentinvention includes the elements of a sensor which contain luminescentindicators. The term “luminescent indicators” refers to molecules,compounds, or substances with the property of emitting light of adefinite wavelength (luminesce) following excitation by a certain amountof energy. Moreover, luminescence indicators have the property that, inthe presence of a particular compound/substance or group ofcompounds/substances, their luminescence is characteristically alteredwith respect to intensity, duration, and/or wavelength. By measuring theluminescence change(s), substances that characteristically alter theluminescence of indicators which are selective for them can bedetermined quantitatively and/or qualitatively as an analyte, dependingon the sensor construction and method of measurement. In principle, allthe luminescence indicators known to specialists can be used in a sensorelement constructed in accordance with the invention. Suitableluminescent indicators include, for example, chemiluminescenceindicators, electro(chemi)luminescence indicators, thermoluminescenceindicators, radioluminescence indicators, sonoluminescence indicators,photoluminescence indicators, and combinations of these. Fluorescentand/or phosphorescent indicators are particularly suitable for use in anelement constructed according to the invention.

The sensor element according to the invention is preferably configuredin such a way that it can be used in optical sensors.

The optical sensor element according to the invention may besingle-layered. If the element is single-layered, the indicators andindicator protectors are contained together in one layer. In a preferredembodiment, the optical sensor element according to the invention has amulti-layered structure, with at least two layers, and preferably two,three, four, five, or six layers, where the indicators and indicatorprotectors are arranged in at least one of the layers of the element.The layers may be constructed and arranged so as to perform variousfunctions within the element. The layers within the multi-layeredelement may differ in terms of layer thickness, composition, and/orconcentration of matrix components and/or the components incorporated init. The segmented, layered structure thus offers the advantage that theelement can be specifically designed for the relevant intended use andadapted to this.

The indicator-bearing and indicator protector-bearing layer or layers ofthe element according to the invention are preferably designed asmembranes, where the thickness of the individual layers, from 0.001 to 1mm, preferably from 0.005 to 0.5 mm, and even more preferably from 0.01to 0.2 mm, is adapted to the element's intended use. Membranes of lowthickness in the range from 10 to 200 microns have the advantage thatthe diffusion rate of the analyte towards the indicators is sufficientlyhigh to permit determination of the analyte, and light emitted from theindicator can reach the sensor's optical detection unit withoutdetrimental transmission losses.

The indicators and the indicator protectors are preferably arranged indifferent layers of the multi-layered, at least two-layered, element,i.e. the element comprises at least one indicator-bearing layer, which(primarily) contains the indicators, and at least one indicatorprotector-bearing layer, which (primarily) carries the indicatorprotectors. The indicators and indicator protectors in the respectivelayers are preferably immobilized, to prevent their diffusion into otherlayers. Immobilization can be achieved, for example, through thecovalent binding of the indicators and indicator protectors to matrixcomponents of the relevant layer. Particular preference is given to adesign in which at least the indicator protector-bearing layer(s) of thesensor element are substantially free of indicators.

In one embodiment, the sensor element according to the inventioncomprises at least one layer comprising indicators and optionalindicator protectors and at least one further layer comprising indicatorprotectors, where the at-least-one further indicator protector-bearinglayer is substantially free of indicators.

In an alternative embodiment, the sensor element according to theinvention comprises at least one layer comprising indicators and atleast one further layer comprising indicator protectors, where theat-least-one indicator-bearing layer is substantially free of indicatorprotectors and the at-least-one indicator protector-bearing layer issubstantially free of indicators.

In a preferred embodiment, the optical sensor element of the presentinvention is designed in such a way that the indicator protectors arearranged in a (wetted) layer (1) that contacts the medium and/orarranged in at least one layer (1 a) of the element that faces towardthe medium and is applied to the wetted layer (2), and the indicatorsare arranged in at least one layer (2) which is applied to the side ofthe indicator protector-bearing layer(s) (1) or (1 a) facing away fromthe medium. Particularly preferable in this case is that the layer (1)and/or the at-least-one layer (1 a) are substantially free ofindicators. In addition to the indicators, the at-least-one layer (2)may also contain indicator protectors. It is preferably substantiallyfree of indicator protectors.

An at least two-layered structure, in which there is at least oneindicator-free or substantially indicator-free indicatorprotector-bearing layer situated upstream from the at-least-oneindicator-bearing layer on the media side, offers the advantage that allof the indicators incorporated in the sensor element according to theinvention receive equally effective protection frominactivating/destructive influences from the medium.

A preferred embodiment of (I) of the sensor element according to theinvention comprises the following layered structure:

-   -   A layer (1) with a side (1.1) that faces the medium and contacts        the medium, and a side (1.2) that faces away from the medium,        situated opposite side (1.1), where this medium-contacting layer        (1) comprises the indicator protectors and is preferably        substantially free of indicators.    -   A layer (2) with a side (2.1) that contacts layer (1) and a side        (2.2) situated opposite side (2.1), where layer (2) is applied        to side (1.2) of layer (1) and includes the indicators and        optional indicator protectors.    -   Optionally one or more further layers (2), which is or are        applied to the already present layer (2) and optionally may        include indicators and indicator protectors.

Optionally at least one further layer (1A) can be arranged between layer(1) and the at-least-one layer (2), which may comprise indicatorprotectors and is preferably substantially free of indicators.

A further preferred embodiment (II) of the sensor element according tothe invention, in contrast to the embodiment (I) described above, isdesigned such that the layer (1) in contact with the media is free ofindicators and also free of indicator protectors and the at-least-onefurther layer (1A) is mandatory.

Type, amount, and distribution of the indicator protectors in the layers(1) and, where applicable, (1A) and (2) of the sensor element may bedifferent. There is preferably a lower proportion of indicatorprotectors contained in layer (2), which also contains indicators, thanthere is in layer (1) and/or in the at-least-one further layer (1A).

Layer (1) and in particular its side (1.1) may be designed as an opaqueoptical isolation layer, in particular when photoluminescence indicatorsare used. Furthermore, the at-least-one further layer (1A) and/or (2)can itself be light-reflecting. This has the advantage of increasing thesensitivity of the measurement when the sensor element is used.

In a particularly preferred embodiment, the element according to theinvention comprises

-   i) an optional, media-contacting opaque layer (1), which is    substantially free of indicators and optionally contains indicator    protectors,-   ii) a second layer (1A), which-   (iia) is applied to the side of the first layer (1) facing away from    the medium,-   (iib) contains indicator protectors,-   (iic) is substantially free of indicators and-   (iid) is optionally light-reflective,-   (iii) a third layer (2) which-   (iiia) is applied to the side the second layer (1A) facing away from    the medium,-   (iiib) contains the indicators,-   (iiic) is substantially free of indicator protectors and-   (iiid) is optionally light-reflective.

To further increase the sensor sensitivity of the optical sensor elementdescribed here, the indicator protector-bearing layer(s) can beseparated from the indicator-bearing layer(s) by at least onelight-reflecting layer, which is neither an indicator protector-bearinglayer or an indicator-bearing layer.

Furthermore, a transparent substrate can be arranged on the side of theindicator-bearing layer facing away from the medium or, in the case ofseveral indicator-bearing layers, on the outermost indicator-bearinglayer opposite the medium. The transparent substrate is transparent tothe light emitted by the indicator or indicators used and alsotransparent to the excitation light, if the excitation of the indicatorsis accomplished using light. The transparent substrate is preferablyselected from glass, plastic, and fiber glass. It is advantageous tocouple the indicator-bearing layer(s) to an optical system via thetransparent substrate. The optical system may be a system known tospecialists, comprising a detection/measurement unit detecting ormeasuring the emission, an evaluation unit, and a means of exciting theluminescence of the indicators, e.g. a light source if photoluminescenceindicators are used.

The layer or layers of the element according to the invention, inparticular the layers (1), (1 a) and (2) and any intermediate layers arearranged in such a way that it is or they are permeable for theanalyte(s). The matrix component of the layer or layers, in particularof layers (1), (1 a), and (2) and any intermediate layers of the elementis preferably selected from a polymer or polymer mixture. In the case ofa multilayered embodiment of the element according to the invention, thematrix part may differ in the individual layers in terms of type,concentration, and/or degree of crosslinking of the polymer buildingblocks. Moreover, within a layer, the concentration and/or the degree ofcrosslinking of a matrix component or the concentration of a componentincorporated in the layer can vary in a gradient manner.

It is particularly preferably that the matrix component is a polymer orpolymer mixture, selected from polystyrene, polyvinyl chloride,polyalkylene methacrylate, in particular polymethyl methacrylate,polyisobutyl methacrylate and poly-2-hydroxyethyl-methacrylate,poly-a-methylstyrene, silica gels, sol-gels, hydrogels, polyurethanes,polytetrahydrofurans, polytetrafluoroethylene, polyester, polybutadiene,polyvinyl butyral, polyethyl acrylate, ethyl cellulose, cellulosetriacetate, cellulose acetyl butyrate, polysulfones, polysulfides andnon-, partially or fully fluorinated silicones and combinations ofthese, optionally in combination with plasticizers.

Layers having light-reflecting properties preferably contain pigmentsand in this case preferably metal oxides such as TiO₂ and Al₂O₃,semi-metal oxides such as SiO₂, and combinations of these.

Opaque layers preferably contain light stabilizers, for example,dark-colored and/or black pigments such as carbon black, graphite,activated carbon, or combinations of these.

As mentioned above, the sensor element according to the inventioncontains indicators which have the property of luminescing afterexcitation, and at least one of their luminescence properties changesmeasurably on contact with one or more analytes. Especially suitable forthe present invention are indicators whose luminescent emission isselectively quenched on contact with at least one analyte.

The invention considers analytes selected from a species of oxygen, inparticular molecular oxygen, carbon monoxide, carbon dioxide, halideions, heavy metal ions, hydroxyl ions, hydronium ions, aromaticcompounds, and combinations of these. However, the sensor elementaccording to the invention is not restricted to the determination ofthese analytes.

In one embodiment, the sensor element according to the inventioncomprises luminescent indicators, in particular photoluminescentindicators the luminescence emissions of which are selectively quenchedby contact with oxygen, and preferably molecular oxygen. Particularlypreferred are indicators the luminescence emissions of which aredynamically quenched in the presence of oxygen, such that contact withoxygen causes the radiationless deactivation of the excited state of theluminescence indicators. Luminescent indicators that are particularlysuitable for the determination of oxygen, including the complexesruthenium, rhenium, rhodium, iridium, and lanthanide, as well asmetallated porphyrins (e.g. platinum and/or palladium-porphyrins),unmetallated porphyrins, or mixtures of these, optionally in combinationwith fluorinated dyes and/or light stabilizers. The luminescentindicators suitable for the various applications are described in detailin the literature.

The proportion of the indicators in relation to the relevantindicator-bearing layer is up to 20 percent by weight, preferably up to10 percent by weight, and more preferably up to 5 percent by weight. Theproportion of the indicators can be adapted to the relevant applicationand may, if necessary, be more than 20 percent by weight.

In the context of the present invention, indicator protectors are ameans to protect the luminescent indicators prior to theirdestruction/inactivation by external influences. The protection of theindicators is achieved by inactivation, neutralization and/or adsorptiveimmobilization of compounds and/or by inactivation,neutralization/insulation of energy-rich radiation, which has aninactivating and/or destructive, especially oxidizing, effect on theindicators.

Suitable indicator protector(s) include reactants of one or morecompounds/substances which act on the indicators in adestructive/deactivating manner upon contact with them. The reactantsreact with the compounds/substances, which have adestructive/inactivating effect, e.g. in a chemical reaction, to becomeat least one product that is harmless for the indicators. The term“indicator protector(s)” in this context is not limited to thereactants, which upon reaction with the compound/substance thatdestroys/inactivates the indicator is converted into at least one newcompound which is not or is less destructive or inactivating for theindicator. The term “indicator protector(s)” also refers to agents whichhave a catalytic effect (catalyst), and which catalyze the conversion ofthe indicator destroying/inactivating compound/substance to at least onenew compound that is not or is less destructive or inactivating for theindicator. In this context, a “catalyst” or “agent of catalyticconversion”, apart from having a purely catalytic effect, also includescompounds that, in addition to the said catalytic effect, also have asimple chemical reactivity, in the sense of a reactant which is itselfconverted during the reaction.

Furthermore, suitable indicator protector(s) include adsorbents of oneor more compounds/substances which have a destructive/inactivatingeffect on the indicators upon contact. Adsorbents completely orpartially inhibit the diffusion of the indicator destroying/inactivatingcompound/substance in the direction of the indicators.

Also particularly suitable as indicator protectors are combinations ofthe above reactants, catalysts and/or adsorbents. It is also possiblethat agents employed as indicator protectors act simultaneously asreactants, catalysts and/or adsorbents.

A sensor element according to the invention which is suitable fordetermining molecular oxygen and/or a sensor element according to theinvention which comprises oxidation-sensitive indicators, preferablycontains indicator protectors, which are selected from

-   -   a) reactants, in particular reducing agents and/or catalysts,        which cause a reduction of strong oxidizing agents,    -   b) adsorbents, which cause chemical and/or physical adsorption        of strong oxidizing agents,    -   c) a combination of (a) and (b).

The term “strong oxidants” refers to compounds/ substances which arecapable of dissociating carbon-carbon single bonds and/orcarbon-hydrogen bonds, and/or in terms their electron donor and electronacceptor properties are comparable with chlorine, ozone, superoxides,hydroxyl radicals and/or peroxide radicals.

A sensor element according to the invention which is designed todetermine molecular oxygen and/or a sensor element according to theinvention which comprises oxidation-sensitive indicators more preferablycontains indicator protectors, which are selected from a) reactants, inparticular reducing agents and/or catalysts for the reduction ofhalogens, in particular chlorine, ozone, hydroxyl radicals, peroxyradicals and/or superoxides,

b) adsorbents for chemical and/or physical adsorption of halogens, inparticular chlorine, ozone, hydroxyl radical, peroxide radicals and/orsuperoxides

c) a combination of a) and b).

As indicator protectors suitable for the reduction of strong oxidizingagents, it is possible, in the sensor element according to theinvention, to use redox-active polymers, which contain oxidizablefunctional groups that can react with strong oxidizing agents, but undernormal conditions, for example in the air, are stable. These polymersmay be incorporated as layers, as particles, or as components ofcopolymer in the indicator protector-bearing layer or layers. Moreover,these polymers can also act both as a matrix material and as indicatorprotectors. In the case of a chemical attack by strong oxidants, thefunctional groups of these polymers are oxidized and thus prevent orreduce the oxidative attack on the indicators, because at least part ofthe oxidizing agent does not reach the indicator molecules.

In a specific embodiment of the invention, the indicatorprotector-bearing layer or layers consist of one or more redox-activepolymer(s) containing oxidizable functional groups, and optionallyfurther containing light protection agents as defined herein.

As indicator protectors with a redox-catalytic and/or adsorptive effect,it is possible, in the sensor element according to the invention, to useagents comprising activated carbon, zeolites, metal oxide, and/ormaterials comprising a semiconductor oxide. These indicator protectorscan be wholly or partially loaded with precious metals such as platinumand/or palladium. Such agents are familiar to specialists in the field.They are capable, if necessary with the help of water, of reducingstrong oxidizing agents such as chlorine, ozone, and oxygen radicalsand/or to slow their diffusion by adsorption. In this case, it isadvantageous for the indicator-bearing layer(s) to be selected frompolymeric matrix materials which permit the presence of water in theimmediate vicinity of the catalyst as a reactant in an adequate, but asmall, amount and which absorb this from the environment.

It is advantageous to select the indicator protectors such that they donot adsorb and/or do not inactivate/neutralize the analyte(s). If thesensor element according to the invention should be suitable, forexample, for the determination of molecular oxygen, then the indicatorprotectors should be selected such that they do not adsorb and/or reducemolecular oxygen.

The indicator protectors are preferably located in the at-least-oneindicator protector-bearing layer in the form of particles having adiameter of 0.1 to 200 microns, more preferably from 0.2 to 100 microns,and most preferably from 0.5 to 50 microns. Depending on the purpose,type, and quantity of the indicator protectors in the at-least-oneindicator protector-bearing layer, the particle sizes may deviate fromthe specified values.

The particle size distribution of the indicator protector particles inthe at-least-one indicator protector-bearing layer is preferably in therange 100 microns or less (100%), more preferably 50 microns or less(100%), and most preferably 30 microns or less (100%).

To effectively protect the indicators from damaging influences, theproportion of the at-least-one indicator protector-bearing layeraccounted for by the indicator protectors is at least 2 percent byweight, preferably from 2 to 75 percent by weight, and more preferably20 to 50 percent by weight. Depending on the purpose and type of theindicator protectors, the proportion of the at-least-one indicatorprotector-bearing layer accounted for by the indicator protectors maydeviate from the specified values. If activated carbon is used as anindicator protector, the proportion of the indicator protector-bearinglayer for which it accounts can be up to 80 percent by weight asappropriate, and preferably up to 50 percent by weight.

Indicator protectors comprising activated carbon are particularlysuitable for protecting the photoluminescence indicators referred toherein from strong oxidizing agents. Particular preference is given toactivated carbon with a BET value of at least 1000 m2/g and/or an iodinenumber of 1000 mg/g. Activated carbon has the advantage that, inaddition to its redox-catalytic and/or adsorptive effect on thesubstances or compounds which destroy/inactivate the indicators, it alsoprovides protection against the ingression of extraneous light from themedia side.

In a particularly preferred embodiment, the sensor element according tothe invention comprises

-   -   indicators selected from photoluminescence indicators as defined        herein, in particular from the complexes ruthenium, rhenium,        rhodium, iridium, lanthanide, of from metallated porphyrins        (e.g. platinum and/or palladium-porphyrins), unmetallated        porphyrins, or mixtures of these, optionally in combination with        fluorinated dyes/or light stabilizers,    -   indicator protectors selected from activated carbon as defined        herein, optionally wholly or partially loaded with precious        metals such as platinum and/or palladium.

If, in the element according to the invention, indicator protectors areused which, when the element is used to determine molecular oxygen in agaseous or liquid medium, cause the reaction of strong oxidants, acertain amount of additional oxygen can occur as the product of thisreaction. To prevent distortion of the measurement of the actualanalyte, the diffusion conditions in the membrane are determined,according to the invention, by the position of the indicator protectorsnear the wetted surface, by their quantity, and by the type andthickness of the polymers used for the individual layers, such that theadditional oxygen produced by the indicator protectors diffuses backinto the solution much faster than it diffuses into the indicators, andthe diffusion gradient within the layer(s) of the element hardlychanges, since differences in concentration in the liquid or gaseousmedium are equalized much faster there, due to the presence ofconvection, than in the membrane phase. The small amount of additionaloxygen generated does not cause any detectable distortion of themeasurement, provided the volume of medium corresponds to the surface ofthe sensor layer in contact with the medium.

A further object of the present invention is a sensor comprising atleast one sensor element according to one of the preceding claims. Thismay further comprise an energy source which is familiar to specialistsin the field and which excited the luminescence emission of theindicators, and may also comprise an evaluation unit.

In a preferred embodiment, the sensor element according to the inventionas described above or a sensor according to the invention comprisingsuch a sensor element for detecting molecular oxygen in a gaseous orliquid medium and/or for determining the molecular oxygen content of agaseous or liquid medium and characterized in that at least oneindicator protector-bearing layer of the sensor element is designed suchthat the diffusion rate of the molecular oxygen formed on the indicatorprotectors by the reduction of a strong oxidant back into the medium isgreater than the diffusion rate of molecular oxygen from the medium inthe direction of the at-least-one indicator molecule-bearing layer.

The sensor element or sensor according to the invention can be used forthe quantitative and/or qualitative determination of one or moreanalytes, and is preferably used for the detection of molecular oxygenin a gaseous or liquid medium and/or for the determination of themolecular oxygen content of a gaseous or liquid medium. The medium,which may include complex media such as waste water or body fluids, cancontain compounds which act on the indicators themselves in aninactivating and/or destructive manner. Since the indicators inside thesensor element according to the invention are effectively protectedagainst inactivating and/or destructive compounds, their presence doesnot result in a greatly shortened service life of the sensor element, asis the case with the sensor elements known from the prior art.

The present invention also relates to a method of quantitatively and/orqualitatively determining one or more analytes in a medium, whichinvolves bringing the medium into contact with a sensor as definedherein, such that this bringing-into-contact takes place on the wettedlayer (1) of the sensor element.

The present invention particularly relates to a method of quantitativelyand/or qualitatively detecting molecular oxygen in a medium, which mayinclude compounds which act in an inactivating and/or destructive, inparticular oxidizing, manner on the indicators themselves, such as, forexample, halogens, in particular chlorine, ozone, hydroxyl radicals,peroxide radicals and/or superoxide. Such strong oxidants are used,sometimes in high concentrations, in waste water or swimming pools fordisinfection purposes. The medium to be analyzed may also be a bodyfluid. The element according to the invention can therefore be used aspart of an in vitro method for determining the oxygen content in theblood.

The method according to the invention preferably further involves thedetection of the analyte(s) by measuring the intensity and/or decay timeof the luminescence emission of the indicators and/or measuring thequenching of the luminescence by means of phase modulation.

The invention described above is explained in more detail below by wayof the example of a sensor element designed for use in an optical oxygensensor, without being restricted to this sensor element.

Thus, the first side (1.1) of the layer (1) of the sensor element thatdirectly contacts the medium may be colored black to absorb light fromthe environment and prevent it from passing into the indicator-bearinglayers. This optical insulation can be implemented by the incorporationof, for example, soot particles or activated carbon particles asdescribed above.

There is at least one, and preferably one, two, or threeindicator-bearing layers (2) applied to the side (1.2) of the layer (1)situated opposite the side (1.1) in contact with the medium. Theindicators are photoluminescence indicators, which are oxygen-sensitive,i.e., their luminescence emission is selectively quenched by contactwith molecular oxygen, preferably the complexes ruthenium, rhenium,rhodium, iridium, lanthanide, as well as metallated porphyrins (e.g.platinum and/or palladium-porphyrins), unmetallated porphyrins, ormixtures of these, optionally in combination with fluorinated dyesand/or light stabilizers. The at-least-one indicator-bearing layer (2)may further comprise indicator protectors.

Optionally, at least one further layer (1A) can be arranged betweenlayer (1) and layer (2), which comprises the indicator protectors and ispreferably substantially free of indicators. Furthermore, theat-least-one further layer (1A) and/or (2) can itself be lightreflective, or they can be separated from each other by at least onelight-reflecting layer, which is neither an indicator protector-bearinglayer nor an indicator-bearing layer, so that more of the excitedluminescence light is reflected onto the photodetector, which increasesthe luminescence output.

A transparent substrate is arranged on the side of the indicator-bearinglayer facing away from the medium, or in the case of severalindicator-bearing layers, on the outermost indicator-bearing layer (2)opposite the medium, which can be made from glass or plastic which ispermeable to excitation light and emission light. Adjacent to this is anoptical system, comprising an excitation light source such as alight-emitting diode (LED), a photodetector, usually consisting ofphotodiodes, corresponding optical filters, and an evaluation unit. Theexcitation and emission light can be transmitted, for example, throughoptical fibers.

Various measurement methods can be used to detect the luminescencequenching by oxygen. It can be carried out using intensity measurements,by measuring the luminescence decay time, or using the so-called phasemodulation technique. The sensory principle, which is based on the phasemodulation technique, consists in the excitation of the luminescence ofthe indicators with a specific excitation light of wavelength v₁, whichis intensity-modulated to a frequency corresponding to its decay time.Due to the modulation of the excitation light, the resultingluminescence of the indicator with wavelength v₂ is also modulated. Fromthe modulations of the excitation light and the luminescence light, aphase shift can be calculated based on the average luminescence lifetimeof the excited states of the indicator molecules. The oxygen diffusingthrough the layer(s) from the medium being measured to the indicatorsaffects the lifetime of their excited states in such a way that there isa radiationless transition of energy from the excited indicators to theoxygen. Corresponding to the ratio of oxygen and excited indicators inthe membrane, the average lifetime is reduced and the determined phaseangle becomes smaller as the amount of oxygen increases. The phasemodulation technique has the advantage that the measured phase angledoes not depend on the signal intensity, provided the luminescenceintensity is sufficiently large to permit a correct measurement with theoptical system. With appropriate calibration, it is possible todetermine the partial pressure of the oxygen dissolved in the mediumbeing investigated from the particular phase angle, using the so-calledStern-Volmer relationship. The partial pressure of oxygen can then beconverted into other physical units.

Essentially responsible for the photo-induced destruction of theindicator is the reactive singlet oxygen, which is produced by theradiationless transition of energy from the indicator to the oxygenmolecule, and chemically attacks the indicator. This can be counteractedby adding light stabilizers such as HALS (hindered amine lightstabilizer) to the indicator-bearing layer, as disclosed, for example,in EP 1757924B1. However, the light stabilizers alone cannot provideadequate protection against e.g. chlorine. For instance, when porphyrinsare used as indicators they are ultimately converted by dissolvedchlorine gas to chlorins and bacteriochlorins, which results not only inthe intensity of the luminescent light decreasing as the exposureincreases, but also in the luminescence light being superimposed and/orabsorbed by reaction products produced by the destruction of theindicator, and the relationship between the phase angle and the oxygenpartial pressure as described above loses its validity. This isparticularly disadvantageous, because the above-mentioned oxidants, dueto their oxidation potential, are used for disinfection purposes inwater treatment, which also precedes all processes in pharmaceuticalbiotechnology and in food production. The sensor element can be exposedto high concentrations of such oxidants, for example, in cleaning anddisinfection processes, and unless the indicators are protected againstthese oxidants it can lose its sensitivity. Furthermore, certainreactions, e.g. in biotechnological processes, which should be monitoredwith the aid of an oxygen sensor, can produce strong oxidants. Anexample of this type of reaction is found in the metabolic process,where the reduction of molecular oxygen in the respiratory chain resultsin the formation of superoxide.

Thanks to the protection of the indicators in the sensor elementaccording to the invention, it is possible to significantly extend itsservice life in the presence of highly oxidative chemical species in atleast partially aqueous measurement media. The protection is achievedhere by the protectors located in the at-least-one indicatorprotector-bearing layer, which cause a total or partial reduction and/oradsorptive immobilization of the oxidative chemical species, and thusprevent or diminish the attack of this chemical species on theindicators.

In the present example, catalyst particles made from activated carbon orzeolites, metal oxides, or semiconductor oxides, with or without aprecious metal loading, for example, of Pt and/or Pd, or other e.g.vitreous carrier materials are used as indicator protectors. In order toachieve a stabilizing effect against oxidative attacks, the quantity ofimmobilized catalyst should be at least 10 percent by weight relative tothe mass of the catalyst-bearing layer. The average diameter of thecatalyst particles is in the sub-millimeter range, preferably in therange from 0.5-50 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a two-layer construction of the sensor element according tothe invention.

FIG. 2 shows a four-layer construction of the sensor element accordingto the invention.

FIG. 3 shows the changes in the phase angle between a sensor elementaccording to the invention and a sensor element known from the priorart, in each case following exposure to chlorine in a chlorine-free andoxygen-free environment.

FIG. 4 shows the changes in the phase angle between a sensor elementaccording to the invention and a sensor element known from the priorart, in each case following exposure to ozone.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a two-layer construction of the sensor element according tothe invention, comprising an indicator protector (•) bearing layer (1)and an indicator (⋄) bearing layer (2). Layer (1) contacts the medium(M), and layer (2) is coupled with the optical system (OS).

FIG. 2 shows a four-layer construction of the sensor element accordingto the invention, comprising an indicator protector (•) bearing layer(1) and an indicator (⋄) bearing layer (2). Layer (2) is coupled withthe optical system (OS). There is a light-reflecting layer (LRS)situated between layers (1) and (2). An optical isolation layer (OIS),which contacts the medium (M), is applied to side (1.1) of layer (1).

EXAMPLE 1

In the outer layer of a three-layer silicone membrane for an oxygensensor, 20 percent by weight, relative to the mass of the wetted layer,of activated carbon particles with a diameter of 10-50 microns wereimmobilized. The procedure for this was as follows:

The prepolymer for the first membrane layer was weighed and mixed with acertain amount of a solvent. Subsequently, the activated carbonparticles were added to the mixture and the entire mass was mixed.Thereafter, the polymer mixture was placed on a smooth surface,generating an approximately 150 micron thick layer. After the solventwas evaporated, the layer was ready to have further layers built up onit. The other layers were constructed in accordance with methods knownto specialists in the field. A small circle of the sensor membranefabricated in this way was bonded to the glass surface of a VisiFermsensor cap, so that the sensory layer of the constructed membrane isfacing toward the excitation light source of the sensor. To test theeffect of the invention, the stability of the sensing membrane describedhere against attack by dissolved chlorine was compared with that of aconventional membrane, such as the one presently marketed by theapplicant for the optical sensor “VisiFerm”.

As described above, the destruction of the luminescence indicator isclearly reflected in the change of the relationship between the measuredphase angle and oxygen partial pressure. This is also particularly truefor the phase angle that is measured in an oxygen-free sensorenvironment, because the luminescence quenching takes place only in thepresence of oxygen. The phase angle determined there is thereforeparticularly suitable for detecting a relevant change in the sensoryproperties and for examining the stability of the sensor membrane. Forcomparison, two identically constructed sensors were used, with onesensor having a conventional membrane, and the other being fitted withthe membrane according to the invention. Both sensors were connected toa data acquisition system for recording the measured phase angle andsimultaneously installed in a reaction vessel filled with 0.1 Mhydrochloric acid. To record the initial situation, the medium in thereaction vessel was first purged with nitrogen to remove dissolvedoxygen from the medium. The corresponding phase angles were measuredwith both sensors. Thereafter, a uniform 0.1% sodium hypochloritesolution was continuously added to the hydrochloric acid by means of adosing apparatus, while constantly stirring, so that both sensors wereexposed to the same definite volume of dissolved chlorine generated bythe procedure. After 15 minutes the supply of sodium hypochlorite wasstopped and the solution was again purged with nitrogen to expel thegenerated chlorine as well as any newly registered oxygen from thesolution. Then the phase angle was recorded again. The chlorine exposureand nitrogenization was repeated four times.

FIG. 3 shows the changes in the phase angle of the two sensors, in eachcase measured after a chlorine exposure in a chlorine-free andoxygen-free environment, in reference to the initial situation. In thecase of the conventional sensor membrane, the phase angle measured in anoxygen-free sensor environment decreased after four exposures to 90% ofits initial value, while in the case of the protected membrane, thephase angle remained almost unchanged. If this phase angle falls byabout 10% or more, the sensor membrane becomes unusable, because theStern-Volmer relation is no longer accurate enough and/or cannot bedetermined accurately enough. This is the case after the fourth chlorineexposure of the conventional membrane. As the comparison clearlydemonstrates, the sensor membrane according to the invention issignificantly more stable against an oxidative attack by chlorine and isstill fully functional after four attacks.

EXAMPLE 2

Analogously to example 1, a sensor membrane was evaluated in regard toits ozone resistance. The sensor membrane was constructed with 25percent by weight activated carbon, relative to the layer in contactwith the medium. To test the effect of the invention, the stability ofthe sensor membrane against an attack by dissolved ozone was compared tothat of a conventional membrane in a manner similar to that describedabove. For this purpose, pure oxygen was passed through one of theozonizers and the gas mixture was directed into a reaction vessel filledwith water, in which a sensor with a conventional membrane and a sensorwith a membrane according to the invention were fitted, for about twohours. As can be seen from FIG. 4, in the case of the conventionalsensor membrane the phase angle decreased to about 95% of its initialvalue following ozone exposure in an oxygen-free environment, whereasthe phase angle remained unchanged in the case of the membrane protectedin accordance with the invention. As this comparison shows, the sensormembrane according to the invention is also significantly more stableagainst an oxidative attack by ozone.

The invention is particularly suitable for the measurement of dissolvedoxygen when strong oxidative substances, such as those used, e.g., fordisinfection, which totally or partially destroy conventional membranesin a short time, are present in the medium.

1. An optical sensor element comprising: at least one indicator composedof a luminescence-active agent; and at least one indicator protector. 2.The optical sensor element according to claim 1, characterized in thatthe element has at least two layers, and the at least one indicator andthe at least one indicator protector are arranged in at least one of theat-least-two layers of the element.
 3. The optical sensor elementaccording to claim 2, characterized in that the at least one indicatorand the at least one indicator protector are arranged in differentlayers of the element, and the element comprises at least oneindicator-bearing layer and at least one indicator protector-bearinglayer.
 4. Optical sensor element according to claim 3, characterized inthat the at-least-one indicator protector-bearing layer is substantiallyfree of indicators.
 5. Optical sensor element according to claim 2,characterized in that the at least one indicator protector is arrangedin a layer of the element that faces towards the medium and the at leastone indicator is arranged in at least one layer (2) which is mountedfacing away from the medium.
 6. (canceled)
 7. Optical sensor elementaccording to claim 5, characterized in that the element comprises i) anopaque layer (1) in contact with the medium, which is substantially freeof indicators, ii) a second layer (1A), which iia) is mounted on theside of the first layer (1) that faces away from the medium, iib)contains indicator protectors, and iic) is substantially free ofindicators, iii) a third layer (2), which iiia) is mounted on the sideof the second layer (1A) that faces away from the medium, iiib) containsthe indicators, and iiic) is substantially free of indicator protectors.8. Optical sensor element according to claim 7, characterized in thatthe indicator protector-bearing layer(s) are separated from theindicator-bearing layer(s) by at least one light-reflecting layer. 9.Optical sensor element according to claim 1, characterized in that theindicators are selected from luminescence indicators, the luminescenceemission of which is selectively quenched on contact with at least oneanalyte selected from oxygen, carbon monoxide, carbon dioxide, halideions, heavy metal ions, hydroxide ions and hydronium ions.
 10. Opticalsensor element according to claim 9, characterized in that theindicators are selected from luminescence indicators, the luminescenceemission of which is selectively quenched on contact with molecularoxygen.
 11. Optical sensor element according to claim 10, characterizedin that the indicators are selected from complexes of ruthenium,rhenium, rhodium, iridium, or lanthanide, or from metallated porphyrins,unmetallated porphyrins, or mixtures of any of the foregoing, ormixtures of any of the foregoing in combination with fluorinated dyesand/or light stabilizers.
 12. (canceled)
 13. Optical sensor elementaccording to claim 1, characterized in that the indicator protectors areselected from a) reducing agents and/or catalysts for the reduction ofstrong oxidants, b) adsorbents for chemisorption or physisorption ofstrong oxidants, or c) a combination of a) and b).
 14. Optical sensorelement according to claim 13, characterized in that the indicatorprotectors are selected from a) reducing agents and/or catalysts for thereduction of halogens, ozone, hydroxyl radicals, peroxide radicalsand/or superoxides, b) adsorbents, for chemisorption or physisorption ofhalogens, ozone, hydroxyl radicals, peroxide radicals and/orsuperoxides, c) a combination of a) and b).
 15. Optical sensor elementaccording to claim 14, characterized in that the indicator protectorsinclude reducing agents selected from at least one redox-active polymerscontaining one or more oxidizable functional groups, activated carbon,zeolites, metal oxide, or semiconductor oxide.
 16. Optical sensorelement according to claim 15, characterized in that the indicatorprotectors are loaded with at least one platinum-group metal. 17.Optical sensor element according to claim 14, characterized in that theindicator protectors do not adsorb or inactivate or neutralize theanalytes. 18.-19. (canceled)
 20. Optical sensor element according toclaim 2, characterized in that the at-least-two layers of the elementdiffer in terms of at least one of their layer thickness and thecomposition of a matrix material.
 21. Optical sensor element accordingto claim 20, characterized in that each of the at-least-two layers ofthe element comprises a matrix material selected from a polymer orpolymer mixture, such that the at-least-two layers differ in terms of atleast one of the type, concentration and/or degree of crosslinking ofthe polymer building blocks. 22.-23. (canceled)
 24. A sensor comprising:an optical sensor element as claimed in claim 7; an energy source thatexcites the luminescent emission of the indicators; and a detection unitoperatively connected to receive the luminescent emission of theindicators.
 25. A sensor element according to claim 13, characterized inthat the at-least-one indicator protector-bearing layer of the sensorelement is configured in such a way that a diffusion rate from theindicator protector-bearing layer back into the medium of molecularoxygen that is formed on the indicator protectors by reduction of astrong oxidant is greater than a diffusion rate of molecular oxygen fromthe medium in the direction of the at-least-one indicatormolecule-bearing layer. 26.-36. (canceled)